It is known that in a given passage, at points which are sufficiently far from the walls of the passage, the stream lines follow paths which are substantially parallel to the walls of the passage formed by the concave and convex surfaces of the blades.
At all points along the path, the centrifugal force which is exerted on a particle is balanced by the pressure forces. The result of this is, generally, that the concave surface of the blade is subjected to a higher pressure than is the convex surface.
It is also known that in the boundary layer near the floor plate and ceiling plate, the speed of the fluid is low. It follows that since the pressure forces are no longer balanced, the stream lines are curves perpendicular to the isobars and follow paths of considerable slippage in each passage from the concave surface to the convex surface as is well known to the person skilled in the art (FIG. 1).
The slippage generates a counterclockwise eddy against the ceiling plate of the passage and a clockwise eddy against the floor plate as seen by an observer placed downstream from the set of blades of FIG. 1.
These disturbances cause important losses known as secondary losses and the smaller the ratio between the height of the blades and the chord, the more the efficiency of a set of blades is reduced.
It has been observed that in the case of a stationary circular set of blades, the effect of the radial static pressure gradient which develops at the outlet end when the meridian line of flow is cylindrical, conical or slightly curved adds to the phenomenon described hereinabove.
This gradient results from the centrifugal acceleration due to the peripheral component of the absolute speed at the outlet end of the blade set and increases the secondary eddy at the upper contour of the flow stream and reduces it at the lower contour thereof (FIG. 2) since the static pressure increases radially from the bottom of the blade set to the top of the blade set.
The variation in the static pressure in the plane between blade sets as a function of the radius is represented by a solid-line curve which continually increases as shown in FIG. 3.
The slope of the curve at the bottom and at the top is equal to: ##EQU1## p: Static pressure in the plane between blade sets. r: Radius.
.rho.: Density of the fluid. PA1 V.sub.u : Tangential component of the absolute fluid speed in the plane between blade sets.
The direction of radial variation of the static pressure which decreases from the ceiling plate to the floor plate amplifies the secondary eddy at the ceiling plate and opposes the secondary eddy at the floor plate, as illustrated in FIG. 2.
In the conventional case of a passage with a floor plate and a ceiling plate, the radial gradient of the static pressure in the plane between blade sets is detrimental at the ceiling plate and favourable at the floor plate. This does not mean, however, that the absolute value of the radial gradient of the static pressure at the floor plate is the best for minimizing the secondary losses.
The invention relates to a turbine stage with a circular stationary blade set followed by a circular moving blade set, each blade set having blades mounted between a floor plate and a ceiling plate which are radially symmetrical about a turbine shaft, the pitch of the stationary blades being L.sub.S at the ceiling plate and L.sub.B at the floor plate and the outlet angle of the stream of fluid from the stationary blade set relative to the plane between said blade sets being .alpha.1.sub.S adjacent to the ceiling plate and .alpha.1.sub.B adjacent to the floor plate, in which stage the distance between the turbine shaft and the surface of the ceiling plate decreases when going from the inlet end of the stationary blade set to the outlet end of the stationary blade set where its value is r.sub.S and then increases going from the inlet end of the moving blade set where its value is r.sub.S up to the outlet end of the moving blade set.
Such a turbine stage is disclosed in British Pat. No. 596 784.
In the stage described in said British patent, the curve of the floor plate and of the ceiling plate is calculated to provide constant pressure in the plane between the blades sets (at the outlet end of the stationary blade set) from the top to the bottom of said space, i.e. the constant radial static pressure gradient is zero.
In the turbine stage in accordance with the invention, the central curve of the ceiling plate at the plane between the blade sets is substantially equal to ##EQU2##
Thus, in the neighbourhood of the ceiling plate, the radial static pressure gradient is equal to the tangential static pressure gradient between the blades sets. This confines the disturbed zone at the ceiling plate to a relatively small flow cross-section.
This invention also relates to a turbine stage with a circular stationary blade set followed by a circular moving blade set having blades mounted between a floor plate and a ceiling plate which are radially symmetrical about a turbine shaft, the pitch of the stationary blades being L.sub.S at the ceiling plate and L.sub.B at the floor plate and the outlet angle of the stream of fluid from the stationary blade set relative to the plane between said blade sets being .alpha.1.sub.S adjacent to the ceiling plate and .alpha.1.sub.B adjacent to the floor plate, in which stage the distance between the turbine shaft and the surface of the floor plate varies continuously from the inlet end of the stationary blade set to the outlet end of said stationary blade set where it reaches a extreme value r.sub.B then varies continuously in the opposite direction from the inlet of the moving blade set where its value is r.sub.B up to the outlet of the moving blade set.
Said turbine stage is also described in British Pat. No. 596 784.